


What Lies Inside the Beast

by EmptyOliveJar



Category: Fire Emblem Series, Fire Emblem: Fuukasetsugetsu | Fire Emblem: Three Houses
Genre: AU where crest bearers can shift into monsters, Implied/Referenced Child Abuse, Knotting, Masturbation, Multiple Orgasms, No beta readers we die like Glenn, Oral Sex, Other, Penetrative Sex, Post Time-Skip, Pretty PWP, but with pre time-skip flashbacks, monster fucking, nb/gender neutral Byleth, wolf beast Sylvain
Language: English
Status: Completed
Published: 2020-10-28
Updated: 2020-10-28
Packaged: 2021-03-08 21:20:01
Rating: Explicit
Warnings: No Archive Warnings Apply
Chapters: 1
Words: 4,287
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/27243328
Author URL: https://archiveofourown.org/users/EmptyOliveJar/pseuds/EmptyOliveJar
Summary: Being born with a crest comes with its own boons and burdens, one being the ability to transform into unique beasts at will. Having lived apart from the world of politics and bloodlines, this strange power some of the students possess is just another part of being a professor to get used to. As one such student, Sylvain should have been one of the easier ones to handle, bright and friendly, if underachieving. Instead, Byleth struggles to resist being drawn into the darkness suffocating him. When the two are reunited after the fall of Garreg Mach, they refuse to deny themselves a second time, especially in the face of Sylvain's monstrous side.
Relationships: Sylvain Jose Gautier/My Unit | Byleth
Comments: 4
Kudos: 22
Collections: FE3H Monsterfucking Weekend 2020





	What Lies Inside the Beast

Winter was riding into Garreg Mach on the malicious wind. If they were blessed with the rotten luck of sleet, they’d have to ground the pegasus and wyvern patrols. The risk would have threatened to unnerve Byleth if it weren’t for the weather’s complete lack of loyalty. If soldiers used to Faerghus’ frigid embrace were struggling to attend to their duties, those from the more temperate Empire didn’t have a chance of advancing. It was perhaps a blessing in disguise, time for their limited troops to rest and formulate a plan.  
  
Despite sleeping for five straight years, Byleth quickly found themselves at their limit upon returning to the monastery. Each day was another brutal cycle of reading lists of the dead, allocating what little resources they had, training their pitiful numbers, and reaching to free Fódlan from the Empire’s grip. Even through the joys of seeing their former students and other allies alive, more than once Byleth had caught themselves yearning to fall back into blissful nothingness. A lifetime ago, they were content with being a semi-literate mercenary. There was hardly even the illusion of choice when Byleth was asked to become a professor, icon, and now, general. Yet the unfortunate truth was that there was no waiting around for someone who wanted the position to take the roles from them, cursing fate and Sothis’s silent absence could only bring so much relief.  
  
It was at least something to know they weren’t the only one unable to shrug off their discontent even as they soldiered on. Some of the tension in their shoulders and brow eased as their eyes fell upon Sylvain, in one piece, breathing, here with them. He was stretched out on the strip of grass outside Byleth’s quarters, unabashedly in his crest beast form. Warmth bloomed inside Byleth’s still chest as memories rekindled.  
  
As much as Byleth tried not to play favorites during their time teaching, it was difficult not to be drawn in by Sylvain’s perplexing combination qualities. That charm and wit violently collided against poorly masked self deprecation. As a student, he was frustratingly unremarkable, earning passing marks while hardly being bothered to study or train. He could have been top of the class if he wanted to, yet Sylvain was picky about the kinds of attention he allowed, brushing away even the smallest praise.  
  
Yet out of all of Byleth’s crest-bearing students, Sylvain demonstrated the most control over his shifting. It was initially a mystery as to how someone so disinterested in expending excessive effort could be so far ahead of even Dimitri, who after receiving only the most prestigious tutelage since birth still struggled to shift into his lion form or back to human at will. Then Sylvain began to drop little details about Miklan leading up to and following Rhea’s orders to apprehend the Lance of Ruin. Byleth didn’t doubt that Sylvain was still with them only because he wrangled control over his crest form so early. Thick fur protected him against the Faerghus chill and a small child had a chance at fighting back against an older brother when he could call forth animal strength.  
  
But here and now, Sylvain wasn’t chasing after some cheap attention and actually appeared to be enjoying a rare moment of stillness, half dozing as the night rolled in. His head rested atop his own fluffy tail, the very same that could fuse with the Lance of Ruin to further bolster his power. His ears twitched as Byleth walked by, offering a rolling woof in greeting. In his own way, he was cute like this, or at least Byleth thought so. His horse-sized form was akin to that of a lanky wolf, fur the same burnished red as Sylvain’s hair. That was where the similarities to the familiar ended, though. Shadows dripped from his maw and festered on the ground he tread. His long limbs were covered in dark, armor-like scales, another jet patch starting behind his ears and draping down his back. Through it all, though, there were the clever amber eyes, recognizable no matter what skin Sylvain wore.  
  
Byleth hadn’t seen him in his crest form since the battle of Garreg Mach, maw bloodied with the viscera of Imperial crest beasts and soldiers. Despite the lasting brand of such images, the vast majority of the time Byleth had spent with Sylvain like this hadn’t been nearly so gorey. In their one-on-one lessons together, Sylvain sometimes shifted for Byleth, offering his assistance in the study of crest shifters in combat in exchange for the forgiveness of his occasional truancy. Robbed of his voice in his crest form, he couldn’t so easily shrug off Byleth’s affirmations and perhaps even dared to enjoy a few of them on his better days, tail twitching with the urge to wag.  
  
If Byleth had to guess, they’d say the beast form was the one thing Sylvain enjoyed about having a crest. For all the power that came with being born with one, the monstrous truth was always looming within. There was a dark irony in that fact being brought to the surface.  
  
Even with a crest of their own, Byleth didn’t pretend to understand what it was like to live under the burden of tradition. The crest of flames was unique, but ultimately, worthless aside from earning them the Sword of the Creator and some favor from Rhea. There was no risk of Byleth being disowned and tossed away for failing to conform to the mold, nobody to curry favor for social gain.  
  
Time spent in his beast form was the closest Sylvain could bring himself to refusing to accept his mantle, still and silent as the breeze plucked at his fur or water rushed over his scales. Always, though, he smothered out that flicker of pure warmth and light, returning to the stygian process of becoming his father.  
  
Despite the unsightly parts of Sylvian and his history, the crest shifter was still a human and a young man at that. Byleth as hardly out of a similar stage of life and had the empathy to politely ignore the often inexplicable bodily reactions that plagued the students. It was all a part of youth, something Byleth repeatedly reminded themselves when their stoicism wavered in the sauna, Sylvain swooning in the heat, shedding what little layers he entered with despite Seteth’s rules against nude steaming.  
  
What Byleth couldn’t pretend wasn’t there, though, was the canid penis that came with Sylvain’s crest form. Even though Byleth was initially left in a stupor at the sight, it only made sense that when Sylvain’s form changed, his entire body was altered. Whether it be a stray breeze or some baser thought that entered Sylvain’s head, without the decency of clothes to hide anything, the ruby tip of Sylvain’s strange member would sometimes poke out from its sheath. It was unfortunate enough to have to deal with the organ in person, but the image followed them out of Sylvain’s training, with it coming a variety of questions that quickened Byleth’s breath. Would others find idea of fucking someone in a crest beast form repulsive? Would it feel like a human cock? Could they even take something that large? Whether or not such questions were meant to deter them, they didn’t remain that way for long.  
  
Whether Sylvain thought much of Byelth’s attempts at discretion or simply desired to poke some fun, a game soon began between them. Sylvain would tease in ways just innocent enough that Byleth couldn’t reprimand him without revealing how much they were reading into their dynamic. Rubbing his face against them was one thing, especially when he didn’t have access to human speech or hand signals while in his crest form. What was quite another and what Byleth had more than once imagined alone at night was their sparring matches. As formidable of an opponent as Byleth knew themselves to be, crest beasts weren’t something they encountered until coming to Garreg Mach. More than once, Sylvain had been able to disarm Byleth with a swing of the tail or sudden rush, but what left Byleth breathless and aching was when Sylvain managed to pin them to the grass, his jaws wrapped around Byleth’s throat before they could so much as thrash.  
  
Though it was obvious he’d won the bout, Sylvain didn’t immediately let go, dagger-like teeth a hair's breadth away from puncturing Byleth’s tender flesh. While any reasonable professor would have reprimanded him then and there, Byleth didn’t protest. They went completely still as the few moments stretched into an infinity, Sylvain’s shadows rolling off their exposed skin like a hellish mist. Byleth always booked the training grounds so it was just them and the student they were working with, but they took extra measures with shifter students, taking them well away from the monastery walls before simulating combat. It started off as a step to reduce collateral damage, but as more and more feral crest beasts surfaced, limiting the sightings of even friendly shifters did its part in reducing anxieties. That also meant Sylvain could do whatever he wanted to them out here, the thought never too far away from the front of Byleth’s consciousness during their sessions. They wondered if he could scent the rising heat between their legs, what he chose to think of it. Right then, Byleth themselves was having a difficult time forming coherent thoughts.  
  
Just because Byleth would never be pushed into the pampered captivity of a pet didn’t mean they couldn’t be taken like an animal or, in the very least, savaged without domestication. And if Sylvain hadn’t chosen to remove his fangs and allow Byleth to gather themselves, they wouldn’t have fought him. Time moved like molasses and lightning all at once and eventually, Sylvain released his hold on Byleth’s neck, staring down his long nose at them as they struggled to pick themselves off the ground. The echoes of his earlier threat to collect Byleth’s unpaid debt to their crest faded into the hills, something equally heavy left in their wake.  
  
But for as cruel and manipulative as Sylvain could be, his heart wasn’t wicked. Byleth certainly remembered how easily they nearly stumbled after him. With as determined as they had been to hide any untoward thoughts towards the students, many of them only a few years shy of Byleth’s own age, Sylvain so readily capitalizing upon Byleth’s latent attraction should have infuriated them. Should have and yet failed to, only because of Sylvain’s honeyed proposal. He was so warm against them as the two danced in the Goddess Tower, moonlight barely able to peep through the dusty windows and the ball’s music muffled through the stone walls. Had Seteth’s watchful gaze been able to reach the courtyard, he’d have given them a stern reminder to leave room for Sothis. Little did he know that Sothis was the one urging Byleth to give Sylvain just one more dance. How easy it would have been in that moment to whisper “yes” against his lips when he asked them to marry him.  
  
If Byleth barely managed to say no to his sudden question, the pull between them, newly open and raw now that Sylvain was here with a man’s body and mind, was insurmountable. It had been easy to avoid being alone with him when both of them were kept so busy, but in the yawning silence of the winter’s night, those unwilling to be swallowed up by the cold and dark scurrying away, there seemed no other choice.  
  
They forced themselves to try to continue on, taking a single step away. That was all the newly awoken, feeling part of themselves allowed before a cold nose butted into the nape of their neck, forcing a shiver through him. Turning, Byleth found Sylvain had silently risen and padded over, putting himself nearly muzzle to nose with Byleth. Shadows pooled around his feet, leaving them tingling even through heavy boots.  
  
Slipping off their gauntlets and letting them drop to their feet, Byleth slowly brought their hands, reddening in their exposure, up to Syvain. It wasn’t until he pushed their head into their grasp that they dared to stroke the sinfully soft fur of his ears. Sylvain blinked languidly as his dark tongue poked out from between his snaggly jaws. Those five years had been kind to Byleth, if only in that they were spared the suffering of fretting over the fate of Sylvain and all the others. There hadn’t been any need for Sylvain to tell Byleth exactly how the time had passed for him. Not when he was waiting for them in front of their quarters, honest in a way he reserved solely for them.  
  
For all the girls he toyed with, Byleth always wondered just how many Sylvain let touch him. Not even in bed, but were permitted to merely take his hand as they walked in the market or sat across a table from one another over tea. Even his childhood friends seemed remiss to acquiesce his motions for casual affection, especially when an ill-deserved slap or verbal rebuke was available. For all Sylvain had done to infuriate and injure, Byleth’s sympathy still lended itself to him and wouldn’t stop for the foreseeable future. He was a kicked puppy that at once cowered from and craved uncertain hands, but damn it, if Byleth didn’t want to keep reaching out.  
  
The last rays of the sun were dissolving over the ruins of the monastery, the shadows skitting across the courtyard wan in comparison to the darkness Sylvain threw off. Byleth leaned into Sylvain’s sturdy body, his strange coat tickling their face as his tail swayed back and forth. He was so warm against them, the air determined to slip its fingers under Byleth’s skin unable to compete.  
  
“You’re stunning.” Byleth murmured to him, fingers slipping up and down Sylvain’s arched neck. “Talented, intelligent, kind hearted. Even like this. Especially like this.”  
  
Byleth started when a wet tongue slid along their jaw, the saliva left behind making their skin prickle. Catching the offending mouth in their hands, Byleth was about to scold him when they paused. As difficult as it was to tell without human words or expressions, his eyes gleamed in a way Byleth hadn’t seen before. Maybe, just maybe, Sylvain was starting to believe those words.  
  
Ever so slowly, Byleth rose to the tips of their toes, pressing a kiss between Sylvain’s glowing eyes. A raspy whine escaped Sylvain’s throat as he lowered himself into a bow to press his upraised head against Byleth’s chest. So they kissed him again on his wet nose, a smile pulling at the corners of their mouth at the speed Sylvain’s tail had achieved. He shook his head free from their grip, offering a wet kiss of his own across Byleth’s lips.  
  
Their mouth parted in a tremulous gaspe, the taste of Sylvain’s tongue fresh on their palette. There was a strange tang to it, like that some of the more twisted reason spells left in their wake. Any interest in making such an analysis drained from Byleth’s mind, however, as blood and rational thought began to divert. Byleth wanted Sylvain more than enough when he was human, but this form carried the taboo allure a young professor’s interest in an older student couldn’t dare to match. The feelings were far from new, but in that moment, every part of Sylvain sang to Byleth, his fur and scales, humid mouth, inhuman cock, and especially knowing the rare satisfaction Sylvain took in this version of himself.  
  
Rhea wanted a miracle of the Goddess from Byleth. Instead, the avatar she so carefully groomed wanted nothing more than to be free of all her pageantry, bare and ready to be debased by the hidden depravity of crests. There was something perversely poetic about it all that Byleth didn’t doubt they would have earned Sothis’s blessing.  
  
The only figures still about were cats and dogs braving the elements for any final handouts, the quarters surrounding Byleth’s either shut up tight or empty. When Byleth went to their door, meeting Sylvain’s eyes as they held it open, there was no more hesitation.  
  
Sylvain’s form seemed to seep into every corner of the room as Byleth drew the lock back into place, the “click” hardly resounding throughout the stripped room before their hands flew to their garments. Buttons flew and laces tore, any desire for spectacle lost in the animal state of both their passions. Byleth hadn’t even gotten their fingers around the waistband of their underwear when Sylvain pounced, knocking them back onto the pile of musty horse blankets that was currently serving as a bed. His claws raked over their hips, drawing thin lines of blood as he ripped away the last bits of cloth between him and Byleth like they were made of tissue.  
  
If Byleth had plans of complaining about one of their only garments being destroyed or the light sting of their wounds, they died from the jolt that shot through them when Sylvain shoved his muzzle between their thighs, his cold nose hit their aching center. Before the shudder could even work its way down their spine, his rough, fiery tongue began lapping at them from ass to tip, bathing them in shadows that seemed to move against Byleth’s skin with their own frigid ardor. Byleth shoved a fist into their mouth to muffle the primal sounds threatening to rip free from within them, the clash of wispy cold and crashing waves of hot nearly enough to drive them mad. He was messy, any technique thrown out in favor of going at Byleth like they were a desert oasis. There was no flirting build into something more tempered, only unrelenting lashings, hitting those most sensitive spots over and over again. Whereas Byleth had been left in a haze after waking, now, every sensation was in shocking clarity, rapturous in its intensity.  
  
Byleth’s fingers gripped the thick fur around Sylvain’s neck, convulsions racking their body as their voice vanished in the torrent. Sylvain didn’t allow their peak to gracefully rise and fall, dragging out spasm after spasm, the height of pleasure stabbing into Byleth and twisting brutally. The sweetest of tortures, the most enthralling death. Their entire body quaked from the catch, drag, and flick of his tongue, even as the floodwaters ebbed into pointed ache. It took Byleth taking Sylvain’s snout in their hands and pulling him away for him to stop and let them breathe again. Dark eyes fixed onto Byleth’s as Sylvain licked his chops, savoring Byleth’s taste.  
  
Mind still clouded even after their pulse had begun to calm, it took a few seconds for Byleth to notice and comprehend Sylvain’s diamond-hard erection, leaking and bobbing in the air as the rest of him stood stone still. With as slick and ready as they were now, any previous intimidation transformed into a determined hunger. There was nothing for them to fear, not when they were with Sylvain.  
  
Byleth shifted their entire body onto the impromptu bed and stuck two fingers into their mouth, liberally coating them with saliva before reaching down between their legs. There was no resistance as Byleth fucked themselves open, that familiar ache coming back and more persistent than ever even after being so recently sated. When they determined they were as ready as they’d ever be, Byleth turned over, getting their knees under them, folding their arms before them to rest their head upon. Even so, their arched back wasn’t taken as the invitation it was. Sylvain jumped onto the bed only to crush his head against Byleth’s, producing a low whine as his eyes met theirs. Byleth reached up, cupping a hand along his furred cheek.  
  
“I’ve got you. Take what you need, Sylvain.”  
  
For a few seconds, neither of them moved or made a sound. Sylvain stepped away and Byleth was about to rise and see what was wrong when his weight settled onto Byleth. There was no time for them to better adjust themselves, Sylvain’s forelegs wrapping around their waist as his pelvis slammed forward, reaching to connect them. The force caught Byleth off guard, arms forced to go from cradling their head to stabilizing their body against being thrown forward. Considerately, the next thrust was gentler, based in exploration rather than frenzy to penetrate as he nuzzled into the side of their neck. The searing tip of his erection brushed against Byleth’s entrance, leaving them keening in want as it slipped to one side rather than entering them. Impossibly hot precome streaked along their ass and the back of their thighs with each failed attempt.  
  
Byleth dared to let their thoughts wander to reaching back and assisting him when Sylvain’s tapered tip finally slipped inside. Byleth’s mewl was drowned out by Sylvain’s triumphant bark, the gravelly sound roaring in Byleth’s ears as each thrust brought even more of Sylvain’s cock, somehow even hotter than his tongue, deeper into them. He’d draw back until his head was inside and then sheath himself inside of them again. The start of the shaft was just barely narrower than the head, his length thickening around the middle before offering a miniscule respite as it thinned out again, forcing Byleth’s hole to stretch and shape itself accordingly with each piston. The constant squirts of precome hadn’t stopped, making each slide smoother, quicker, farther. Soon, the knot began to bash against their entrance and they did their best to relax, needing Sylvain in his entirety.  
  
The first part of the knot to slip into Byleth left them wincing around the stretch. It was nearly too much then and it had still yet to inflate to its fist-like full size. If they weren’t sopping wet long before entry, they would never have even gotten this far. But the knot was nearing its midway point with each punishing thrust, Byleth’s pride refused to allow them to stop. Using what little leverage they had, Byleth pushed back, hips coming back in time to meet with Sylvain’s. Any earlier discretion between them was gone, the allegro beat of Sylvain’s body against theirs reverberating around the room.  
  
All of the air in Byleth’s lungs left them in a sharp gasp at the pop of the knot fully entering. They’d hardly drawn in a stuttering breath when, so full they were no longer sure where Sylvain ended and they began, when it was pulled out and shoved back in again. All Byleth could do was squeeze their eyes shut, caught between needing the agonizing pleasure to end and never wanting it to cease. Somehow, Sylvain’s knot was still expanding, extracting wails and pleas from Byleth as he snarled in his struggles to continue pumping in and out. Byleth into a broken scream, desperate for a second orgasm to distract from this overwhelming intrusion as they were tied together.  
  
Heaving breaths and inky drops fell from Sylvain’s open maw as his cock jerked and twitched inside of Byleth, releasing jets of searing come, filling what little room Byleth had left. His knot acted as a perfect seal between his ejaculate and the outside world ensuring Byleth received all he had to give. All Byleth could do was whimper through it all, that orgasm they so badly yearned for hanging over them like a threat, the knot firmly pressed against that sponge with nowhere to go. Sylvain was too preoccupied to offer support or snide satisfaction at Byleth’s state, still coming in white hot spurts inside them. Despite the earlier release leaving the sensation of touching themselves further akin to an electric shock, it was preferable to leaving the inflated bulb pressed up against that tenderest place, twitching erratically with each fresh pump of semen. Tears dewed in Byleth’s eyes, walls clenching to drive Sylvain into the ruinous spot within them as they furiously worked their hand against the sting. Though Sylvain whined at the added stimulation, he let Byleth continue, head craning to lick at their face in encouragement.  
  
At last, the pressure building inside Byleth’s abdomen gave way, every thought and sensation outside of coming ceasing to exist. In those rapturous seconds, the only things in the world that mattered where the two of them, simply as bodies connected so intimately. Byleth wasn’t a mercenary or a professor, Sylvain man nor beast. They simply were, lost in one another despite what might lay outside that room.  
  
When Byleth grew slack, Sylvain turned, shifting his weight so he was no longer crushing them under him. His knot still remained firmly wedged inside of them and Byleth could only wonder at how long it would take to deflate, but there were far worse things than being trapped against Sylvain. He began to turn, waiting for Byleth to follow the movement so they were both lying on their sides. Numb after such a rush of sensation, the shift hardly elicited a squeek from Byleth, their body as acclimated to the alien presense as it could be. All the same, Sylvain curled around Byleth with a sigh, his warm breath chasing away the dark mist before it could sink into their skin. The sweat coating Byleth was losing its heat and they gratefully burrowed into Sylvain’s fur. A full tail draped over the rest of their flesh, more luxurious than anything in Rhea's quarters. They wondered if he would stay after the tie between them released, if he could ever let them touch his true skin.  
  
Time wasn’t a luxury war afforded, yet Byleth found themselves willing to wait and see if the man and the beast would ever come into harmony, neither side enough to frighten them away.

**Author's Note:**

> Thank you so much to the mods of FE3H monsterfucking weekend! I can't think of anything more tailored to my interests. 
> 
> Kudos and comments are deeply appreciated! 
> 
> If you're interested in my crest shifter AU or have something similar of your own, I'd love to chat about it!


End file.
